I'm 29 weeks and 4 days. Today at the ultrasound they said my baby mea…

I'm 29 weeks and 4 days. Today at the ultrasound they said my baby measures 32 weeks. Yet when my period was late by a month the pregnancy wasn't detected by either a test, an hCG blood test, or an ultrasound. Could this be possible, and how can I now check the exact gestational age?